In order to reveal the mechanical deterioration characteristics and stress–strain response mechanism of remolded moraine soil subjected to freeze–thaw cycles were investigated using moraine soil collected from Danniang Township on the southeastern margin of the Qinghai–Tibet Plateau. A series of laboratory freeze-thaw cycle tests, triaxial shear tests and mercury intrusion porosimetry(MIP) tests were conducted to systematically investigate the effects of confining pressures (50 kPa, 100 kPa, 200 kPa) and freeze-thaw cycle numbers (0, 2, 5, 10, 15 and 20 cycles) on the deterioration characteristics and failure modes of remolded moraine soil. The modified Duncan-Chang model with soil softening parameter was introduced to accurately characterize the stage-dependent evolution of the stress–strain behavior of the remolded moraine soil. The results indicate that the stress–strain curves of the remolded moraine soil under freeze–thaw cycling exhibited typical weak strain-softening characteristics, and the peak strength evolution followed a three-stage pattern characterized by rapid reduction, gradual variation, and eventual stabilization with increasing freeze–thaw cycles. The reductions in peak strength between successive freeze–thaw intervals were 4.64%, 7.04%, 7.16%, 3.60%, and 1.60%, respectively, whereas the corresponding reductions in the initial elastic modulus were 5.37%, 16.83%, 12.76%, 6.81%, and 3.14%, respectively. Furthermore, the modified Duncan–Chang model incorporating the softening parameter was shown to effectively reproduce the weak strain-softening process. The findings provide new insights into the instability mechanisms of disturbed moraine soil slopes and the fragmentation mechanisms of debris-flow source materials in the southeastern margin of the Qinghai–Tibet Plateau, and may offer scientific guidance for geological hazard prevention in regional urban development and engineering construction.